1. Field of the Invention
The present invention relates to a potential measuring apparatus for detecting a potential of a detection object through a quantity of electric charge generated in a detection electrode. The present invention also relates to an image forming apparatus equipped with this potential measuring apparatus.
2. Related Background Art
Japanese Patent Application Laid-Open No. 2000-180490 has proposed a potential measuring apparatus of a system that a distance between a-detection object and a detection electrode is changed. This potential measuring apparatus comprises a piezoelectric tuning fork, an insulator and a detection electrode and has the construction of the so-called capacitor in which the insulator is formed between the detection electrode and the piezoelectric tuning fork. In this construction, electric charge is generated, increased and decreased at the detection electrode whose distance to the detection object is changed, and this increase and decrease of electric charge becomes an AC electric signal from the detection electrode.
U.S. Pat. No. 6,177,800 has proposed a potential measuring apparatus of a system that an area of a detection electrode as seen from a detection object is changed. This potential measuring apparatus is produced by the MEMS technique (application of a semiconductor process technique). In semiconductor processes, silicon is generally used as a substrate material. When the detection electrode is formed on silicon, an insulator is formed between the silicon and the detection electrode. Since silicon has a nature that electricity is passed through it, the so-called capacitor is constructed. In this construction, electric charge is generated, increased and decreased in the detection electrode, of which the area seen from the detection object is changed, and this increase and decrease of electric charge becomes an AC electric signal from the detection electrode.
The principle of generating an output signal in a potential measuring apparatus of a non-contact system, which is a system used in the potential measuring apparatus of the prior art and the potential measuring apparatus according to the present invention, will hereinafter be described.
When a distance (g) between the detection object and the detection electrode, a dielectric constant (∈) between the detection object and the detection electrode, or an area (s) of the detection electrode as seen from the detection object is changed, an electric (coupling) capacitance (C) induced between the detection object and the detection electrode is altered.
The electric capacitance (C) may be generally represented by the following equation (1):C=(∈·s)/g  (1)wherein ∈[F·m−1] is a dielectric constant between the detection object and the detection electrode, g [m] is a distance between the detection object and the detection electrode, and s [m2] is an area of the detection electrode as seen from the detection object.
The electric capacitance (C) may also be represented by the following equation (2):Q=C×Vd  (2)wherein Q is a quantity of electric charge, and Vd [V] is a potential of the detection object.
The equation (1) is substituted into the equation (2) to obtain the following equation (3):Q=(∈·s)/g×Vd  (3)
When the area of the detection electrode as seen from the detection object is changed with time (t), the equation (3) can be represented by the following equation (4). This change can be attained by, for example, letting in and out a shield plate formed of a conductive material or the like between the detection object and the detection electrode.Q(t)=(∈·s(t))/g×Vd  (4)
The equation (4) is differentiated with respect to the time (t) to obtain the following equation (5):dQ(t)/dt=I(t)=(∈/g·ds(t)/dt)×Vd  (5)wherein the area change per time, ds(t)/dt, is a known value.
In this way, a current signal I(t) from the detection electrode is obtained in accordance with the equation (5). This signal is subjected to current-voltage conversion, whereby a voltage output signal V(t) can be obtained, and the potential Vd of the detection object can be found from the output signal V(t). When the distance (g) between the detection object and the detection electrode is changed with time (t), or a dielectric constant (s) between the detection object and the detection electrode is changed with time (t), it is also understood that a current signal I(t) from the detection electrode is obtained in accordance with the same way of thinking as described above.
However, in the potential measuring apparatus of the non-contact system, an electric capacitance (an electric capacitance exclusive of the electric capacitance generated with respect to the detection object; hereinafter also referred to as a parasitic capacitance) is generated between the detection electrode and a member present in the vicinity thereof. In some cases, a part of the electric signal generated at the detection electrode may flow into the member present in the vicinity of the electrode by this parasitic capacitance. Therefore, there is a possibility that the output signal from the detection electrode may be lowered. If a noise component is present in the member present in the vicinity of the detection electrode, the noise component flows into the detection electrode, so that the signal-to-noise ratio (S/N ratio) may be lowered in some cases. In the above-described prior art, this has not been taken into consideration.